The present disclosure relates generally to detection of solid materials and, more particularly, to a method and system for detection of solid materials in semiconductor manufacturing apparatuses and processes using an electromagnetic circuit, such as a microwave circuit or a radio frequency (RF) circuit.
Recently, much attention has been focused on developing low-k dielectric thin films for use in the next generation of microelectronics. As integrated devices become smaller, the RC-delay time of signal propagation along interconnects becomes one of the dominant factors limiting overall chip speed. With the advent of copper technology, circuit resistance has been pushed to its practical lowest limit for current state of the art, so attention must be focused on reducing capacitance. One way of accomplishing this task is to reduce the average dielectric constant (k) of the thin insulating films surrounding interconnects. The dielectric constant of traditional silicon dioxide insulative materials is about 3.9. Thus, lowering the dielectric constant below 3.9 will provide a reduced capacitance.
Low-k dielectric materials used in advanced integrated circuits typically comprise organic polymers or oxides, and have dielectric constants less than about 3.5. The low-k dielectric materials may be spun onto the substrate as a solution or deposited by a chemical vapor deposition process. Certain low-k film properties include thickness and uniformity, dielectric constant, refractive index, adhesion, chemical resistance, thermal stability, pore size and distribution, coefficient of thermal expansion, glass transition temperature, film stress, and copper diffusion coefficient.
In fabricating integrated circuits on wafers, the wafers are generally subjected to many process steps before finished integrated circuits can be produced. One such process step is what is known as “ashing.” Ashing refers to a plasma-mediated stripping process by which photoresist and post etch residues are stripped, ashed or removed from a substrate upon exposure to the plasma. The ashing process generally occurs after an etching (front-end-of-line or back-end-of-line) or implant process (front-end-of-line) has been performed in which a photoresist material is used as a mask for etching a pattern into the underlying substrate, or for selectively implanting ions into the exposed areas of the substrate. Any remaining photoresist (as well as any post etch or post implant residues on the wafer after the etch process or implant process is complete) must be removed prior to further processing, for numerous reasons generally known to those skilled in the art. The ashing step is typically followed by a wet chemical treatment to remove traces of the residue, which can cause further degradation or loss of the underlying substrate. In the case of low-k dielectric substrates, material degradation, loss of material, and an increase in the dielectric constant may also result.
Ideally, the ashing plasma should not affect the underlying low-k dielectric layers, and preferably removes only the photoresist material. The use of silicon dioxide (SiO2) as a dielectric material has heretofore provided high selectivity with respect to traditional ashing gas sources. Typically, the ashing or burning of organic material, such as photoresist on a semiconductor substrate like silicon (Si) is implemented through the use of an oxygen or nitrogen-based plasma chemistry. However, low-k dielectric materials, especially carbon containing low-k dielectric materials, can be sensitive to this type of process step. More specifically, the conventional plasma chemistry used during ashing can strip both the photoresist materials, as well as remove a portion of a low-k dielectric film.
One approach that implements an oxygen and nitrogen-free photoresist removal chemistry is disclosed in U.S. application Ser. No. 09/855,177, assigned to the assignee of the present application, and incorporated herein by reference in its entirety. As discussed therein, while the oxygen and nitrogen-free chemistry is not damaging to the low-k material and can successfully remove the photoresist, the actual mechanism of photoresist removal is different from that of the conventional oxygen or nitrogen based ashing chemistry. Conventional ashing is a process in which the photoresist is typically completely volatilized into OH and CO, which emit light at their characteristic wavelengths. As such, an endpoint of the ashing process may be detected by simply monitoring the intensity of these light emissions until they disappear.
In contrast, the removal mechanism discussed in the '177 application (e.g., plasma formed from helium and hydrogen) involves a partial sublimation of the photoresist material, which thereafter has a tendency to rapidly re-solidify. The net effect is the introduction of solid and semi-solid materials into the gas as it gets pumped out. However, since there is no light emitted in this type of a reaction, end-point detection becomes a difficult task. In addition, as a result of the mechanism of removal, the plasma exposure tends to deposit large amounts of the sublimed photoresist and byproducts within the processing chamber and in areas downstream from the plasma process chamber, such as in the throttle valve and exhaust lines.
Accordingly, there exists a need for detecting the presence and relative amounts of solid material and semi-solid material entrained in a medium, such as sublimed photoresist in a helium-hydrogen gas mixture, resulting from removal processes suited for low-k applications.